1. Field of the Invention
The disclosure relates to a sidelight planar lighting device.
2. Description of the Related Art
In order to further improve the visibility of display screens, there have recently been an increasing demand for planar lighting devices (backlight) used for liquid-crystal display devices and other devices to have higher luminance. In a conventional pattern shape of connection wiring formed on a circuit substrate on which light-emitting diodes (LEDs) serving as light sources are mounted, however, there is a limit to the distance between mountable LEDs and thus to the number of LEDs.
Japanese Laid-open Patent Publication No. 2013-149511 is disclosed as an invention described in a publication relating to the present disclosure.
In order to further increase the luminance in a planar lighting device, it is necessary to further increase the number of LEDs arranged along a side end surface (light entrance surface) of a light guide plate. In other words, it is necessary to make a pitch between the LEDs narrower.
FIG. 6 is a plan view of an arrangement of lands in a circuit pattern of a light source unit of a conventional planar lighting device and electrode terminals in an LED package connected to the lands. In the conventional sidelight planar lighting device, a plurality of LED packages (not illustrated) are mounted on a substrate 102. The LED packages are arranged in a manner abutting on a side end surface of a flat light guide plate (not illustrated). The LED packages output light to the inside of the flat light guide plate through the side end surface thereof.
The substrate 102 on which the LED packages are mounted is flexible printed circuits (FPC), and the surface thereof is covered with a cover lay 104 made of an insulating material. A pair of electrode terminals t101 and t102 of the respective LED packages is connected to lands 102r. The lands 102r and a part of connection wiring (conduction pattern) connected to the lands 102r are exposed from openings 104k of the cover lay 104. In other words, a part of the connection wiring that couples the facing lands 102r of the adjacent LEDs is covered with a part of the cover lay 104.
In the conventional technique, as illustrated in FIG. 6, a gap (clearance) s10 between the respective lands 102r and the electrode terminals t101 and t102 on the inner sides of the two electrode terminals t101 and t102 in the LED is reduced to 0.1 mm. The mounting accuracy of the LED is thus secured.
In the conventional technique, a gap s11 (=0.29 mm) between the outer sides of the electrode terminals t101 and t102 of the LED package and the outer sides of the respective lands 102r of the substrate 102 is set larger than the gap s10 (=0.1 mm) between the inner sides of the electrode terminals t101 and t102 of the LED package and the inner sides of the respective lands 102r of the substrate 102. In other words, in the conventional technique, a pair of lands 102r is formed such that the pair of electrode terminals t101 and t102 of the LED package is arranged at an inner area in the pair of lands 102r of the substrate 102. Protruding portions t101c and t102c of the electrode terminals t101 and t102, respectively, are arranged in the pair of lands 102r (rectangular main lands) of the substrate 102.
To increase the luminance of the LEDs as described above, there are two following requirements. First, it is necessary to mount the LEDs with as high density as possible while considering limiting conditions, such as wiring.
Second, to suppress light leakage when LED light enters into the light guide plate, it is necessary to improve the mounting accuracy of the LEDs. Specifically, to stably provide the lighting properties (high luminance and uniformization in lighting) of the planar lighting device at a high level, it is necessary to fix all the LEDs to predetermined positions with high accuracy. To achieve this, it is necessary to perform self-alignment with high accuracy when the LEDs are mounted on the lands 102r of the substrate 102.